Biology Unit 5: Forces of Evolution

The forces of evolution include mutation, gene flow, natural selection, and genetic drift. Co-adaptation and co-evolution, anthropogenic activities, extinction (briefly), and periodic/mass-scale causes and events are also considered.

There are four major forces of evolution: mutation, gene flow, natural selection, and genetic drift.
Mutation introduces new alleles into a population due to errors in DNA replication (copying) and transcription.
Gene flow occurs through migration and contributes alleles between populations.
Natural selection favors individuals with traits that increase survival and reproduction. Differential survival and reproduction success drive this process.
Genetic drift is due to random sampling in small populations and leads to allele frequency changes.
These forces alter allele distributions, turnover, and diversity within populations.
They cause deviation from Hardy-Weinberg equilibrium and are thus responsible for evolution.
Interactions contribute to variation and spread of alleles within/between populations.

Acts as raw material for evolution, occurring in every generation.
Mostly random changes in phenotype (color, size, shape) or genotype (DNA sequence).
Introduces new alleles and alters allele frequencies in a population.
Survival of new alleles depends on their effect on population survival fitness.
If an allele is advantageous, it's favored by natural selection.
Vernon Ingram identified a change in the amino acid (from glutamic acid to valine) in mutant haemoglobin causing sickle-cell anemia.
Mutation frequency in humans is ~1 in 500 million nucleotides (Sudi and Ali-Dunkrah, 2005).
Mutation types categorized by frequency, mechanism and location:
Variants (frequency < 1%), polymorphisms (≥1%), single nucleotide polymorphisms, indels, tandem repeats, and copy number variations.
Mutation rate is low and varies across genome sites.
Some sites have higher mutation frequencies (hotspots).
Mutations in mitochondrial non-coding regions (HV1 and HV2) are used to study maternal history.
Spontaneous mutations rate ranges from 1 in 104 - 108 genes per generation.
Alu transposition elements (short interspersed elements or repeats) occur frequently in every new birth.
Inborn errors of metabolism (IEMs): Several types of IEMs exist, arising from mutations.

Mutations cause genetic disorders (e.g., sickle cell anemia, thalassemia, cystic fibrosis, Huntington's disease) and cancer.
Affected genes can be autosomal (single genes), sex-linked, or involve chromosomal abnormalities. Examples of genetic disorders: mutations in BRCA1 and BRCA2 (tumors suppressor genes), leading to breast, ovarian, pancreatic, and prostate cancers. Specific syndromes and diseases are also listed in the text.

Carriers of sickle cell trait (HbAS) are protected against malaria.
Other mutation examples are given in the text (e.g. lower incidence of cardiac complications in sickle cell anemia with alpha thalassemia).

Silent mutations do not affect the organism.
Can affect the splicing functions of exons, affecting the sequence of nucleotides.

Movement of alleles between populations.
Affected by socio-economic factors (e.g., migration, mate selection, interbreeding).
Causes changes in allele frequencies in both the moving and recipient populations.
Examples include Anglo-Indians, American Blacks, and Siddis (Indo-African population) and Latin American populations.

Models explain allele frequency changes due to gene flow: 'island', 'isolation by distance', and 'stepping stone'.
Stepping stone populations exchange migrants with immediate neighbors.
Island populations exchange migrants with other populations.
Isolation by distance reduces gene flow as geographical distance increases.

Differential survival and reproduction success.
Determined by an allele's suitability to the environment.
Negative selection eliminates unsuitable alleles (purifying selection).
Positive selection, or diversifying selection, supports advantageous mutations, promoting the allele.
Features of natural selection: survival, reproductive success, fitness/adaptive value, mate selection, female fecundity, survival in reproductive age.
Skin color and high altitude adaptations are given as examples.
Examples: skin color, physiological and genetic adaptation to high altitude, lactose persistence, sickle cell trait, Duffy blood group.
Mutations resulting in specific traits (e.g., lighter skin to absorb vitamin D in some populations).
Adaptation to high altitude, resulting in increased red blood cell numbers greater lung capacity, and chest dimension.

Selection for heterozygotes
Selection against heterozygotes (underdominance)
Selection with co-dominant allele (e.g., ABO blood types).
Selection against dominant allele (e.g., achondroplasia)
Selection against recessive homozygotes (e.g., Tay-Sachs disease)
These are further detailed concerning specific criteria, examples, and implications.

Bottleneck effect: Population reduction due to a catastrophic event. Surviving individuals have different allele frequencies than the original population.
Founder effect: A few individuals start a new population. The founder population's allele frequencies may differ significantly from the original population.